This study aims to assess the safety, tolerability and Pharmacokinetics/ Pharmacodynamics (PK/PD) profile of three doses of MOD-4023 on a weekly regime and one dose on an every-other-week regime administered for a period of 4 weeks in Growth Hormone Deficient Adult (GHDA) patients who previously were on a stable r-hGH treatment. An additional extension period of 16 weeks once-weekly administration of MOD-4023 aims to confirm the dose selection for future trials.